1. Field of the Invention
The present invention relates to an inherently open-celled polypropylene-based polymeric foam having a large (at least 1.5 millimeter) average cell size, a process for preparing such a polymeric foam and a process for using such a polymeric foam.
2. Description of Related Art
Foams having a large average cell size (that is, average cell size of at least 1.5 millimeters) are desirable in applications such as filtration and acoustical damping. These same applications benefit from an open-celled foam. Frequently, these applications would further benefit from thermal stability associated with polypropylene (PP) foams so as to allow filtration of hot liquids or acoustical dampening in warm places (for example, engine compartments).
PP foams are desirable for their thermal stability, ability to be easily made by extrusion processes, and recyclability. However, preparing PP foams that have a large average cell size and that are inherently open-celled is challenging. A foam is “inherently open-celled” if it becomes open-celled as the foam expands during formation, as opposed to, for example, during a mechanical perforation step.
U.S. Pat. No. 6,590,006 B2 discloses a polyolefin foam that has a large cell size that is made from a combination of at least 35 percent by weight (wt %) high melt strength (HMS) PP and up to 65 wt % of a free radical polymerized ethylene polymer. However, U.S. Pat. No. 6,590,006 B2 teaches how to prepare a close-celled foam and then perforate the foam to achieve open-cell character.
U.S. Pat. No. 6,541,105 (U.S. Pat. No. '105) discloses open-celled polyolefin foams including an open-celled PP foam that has a cell size of greater than one millimeter, but requires blending polyolefins that have a melting point difference of at least ten degrees Celsius (° C.).
It is desirable to advance the art of PP foams to include PP foams having a cell size larger than that obtainable with the U.S. Pat. No. '105 technology. It desirable to have a PP foam that is inherently open-celled so that perforation is not necessary. It is even more desirable if the polymer blend forming the foam did not need to comprise polymers containing a melting point difference of at least ten ° C. Still more desirable is a polymer composition that can be blown into an inherently open-celled PP foam over a die temperature window of at least ten ° C.